Something to think about and implement if possible for future reference - a potential roadmap:

One option could be to create a function that allows players to sell multiple items at once with just one click.

This could involve creating a new user interface that allows players to select which items they want to sell,
and then automatically calculates the sale price based on the items selected.

Another option could be to add more functionality to the existing sellable items feature. 
For example, letting players haggle with merchants to get a better price for their items, or allow them to bundle items together to create special deals.
 
1. Add a crafting system: Allow players to break down items they don't need into crafting materials, which they can then use to create new, more powerful items. Introduce rare crafting materials that can only be obtained through difficult quests or boss battles. 

2. Implement item sets: Create sets of items that provide bonuses when worn together. 
For example, wearing a full set of armor might provide increased defense, while a full set of weapons might provide increased damage. This very well may involve using more trinket slots (up to four).

3. Allow players to upgrade items: Introduce an upgrade system that allows players to spend gold and crafting materials to increase the stats on their favorite items. This could involve adding special modifiers, increasing damage or healing, or adding unique abilities. 

What does everyone else think about these features?